Mechanical properties of Cu‐based Micro‐ and Macrocomposites

There is a need for high‐strength and highly‐conducting materials for applications such as pulsed high magnetic field coils. Two different approaches were studied in order to strengthen copper‐based conductor materials. On the one hand, microcomposite Cu‐Ag alloys yield high strength as a consequence of their nanoscale microstructure and, on the other hand, a Cu‐based macrocomposite can be strengthened by the use of a steel jacket. In both cases the increase of strength coincides with a decrease of conductivity. Thus, the ideal material balances between these two competing properties.     

Holographic study of conventional and negative Poisson's ratio metallic foams: elasticity,yield and micro-deformation

An experimental study by holographic interferometry is reported of the following material properties of conventional and negative Poisson's ratio copper foams: Young's moduli, Poisson's ratios, yield strengths and characteristic lengths associated with inhomogeneous deformation. The Young's modulus and yield strength of the conventional copper foam were comparable to those predicted by microstructural modelling on the basis of cellular rib bending. The re-entrant copper foam exhibited a negative Poisson's ratio, as indicated by the elliptical contour fringes on the specimen surface in the bending tests. Inhomogeneous, non-affine deformation was observed holographically in both foam materials.     

Mechanical-Property Changes of Polymeric and Composite Materials after Low-Temperature Proton Irradiation

The mechanical properties of polymeric and composite materials are known to be sensitive to ionizing radiation. Most of the existing data, however, is the result of near-room-temperature irradiations, most commonly with ⁶⁰Co gamma irradiation. For use of these materials in applications such as for magnetic fusion magnets, where operation will be at cryogenic temperatures in sometimes severe radiation fields, knowledge of the materials’ radiation response to low-temperature irradiations is required. This paper reports the results of mechanical-property-change measurements made at 4.2K on a number of potential magnet materials following 200-MeV-proton irradiation at temperatures below 20K. Standard three-point bend tests were performed at 4.2K for short-beam shear determinations in the laminate materials and for shear strength in the remainder of the specimens. Specimens were warmed to room temperature for one week prior to the mechanical testing in order to emulate the expected mechanical state of the material assuming room-temperature cycling in the expected magnet applications. Data are presented in the form of yield stresses before and after irradiations with percentages of change. There were five specimens per test dose for each material. Data are presented for exposures ranging from nominally 10⁷ to 10⁹ rad. Results of the mechanical tests range from complete delamination and distortion of the specimens at 10⁹ rad to an increase in the yield stress of 63% after 10⁹ rad. The latter specimen did, however, evidence significant embrittlement. The phenomenon of irradiation-induced strengthening due to enhanced cross linking in undercured polymers was observed in some cases.     

Label-free identification of single dielectric nanoparticles and viruses with ultraweak polarization forces

Label-free detection of the material composition of nanoparticles could be enabled by the quantification of the nanoparticles' inherent dielectric response to an applied electric field. However, the sensitivity of dielectric nanoscale objects to geometric and non-local effects makes the dielectric response extremely weak. Here we show that electrostatic force microscopy with sub-piconewton resolution can resolve the dielectric constants of single dielectric nanoparticles without the need for any reference material, as well as distinguish nanoparticles that have an identical surface but different inner composition. We unambiguously identified unlabelled ~10 nm nanoparticles of similar morphology but different low-polarizable materials, and discriminated empty from DNA-containing virus capsids. Our approach should make the in situ characterization of nanoscale dielectrics and biological macromolecules possible.     

Corrosion fatigue behaviour and fracture mechanisms in nitrided low alloy steel

Rotary bending fatigue tests have been performed in 3%NaCl aqueous solution using specimens of a low alloy steel (Cr–Mo steel) with different nitride case depths. The effect of case depth on corrosion fatigue strength, the fracture process and mechanisms were studied. The corrosion fatigue strengths of the nitrided materials increased compared with the untreated material and increased with increasing thickness of the compound layer, but tended to saturate above a certain thickness. All the materials showed lower fatigue strength in 3%NaCl aqueous solution than in laboratory air and the reduction of fatigue strength decreased with increasing thickness of the compound layer, but remained nearly constant above a certain thickness. Corrosion pits were seen underneath the compound layer, from which cracks initiated. The corrosion fatigue strengths of the specimens whose compound layer was completely removed by electropolishing were almost the same as that of the untreated material, indicating a very significant role of the compound layer in improving corrosion fatigue strength. Because of the porous nature of the compound layer, particularly in the surface‐adjacent part, the solution penetrated the compound layer and reached the base steel, thus the corrosion fatigue strength of the nitrided materials was controlled by the penetration of corrosive media.     

Fatigue strength of high strength dual-phase steel sheet

Fatigue tests have been carried out on lean-alloyed dual-phase steels with tensile strengths ranging from 300–800 MPa. Smooth specimens and specimens with punched holes were tested. The fatigue strength of dual-phase steel was found to be similar to that of other types of steel (eg solution hardened or microalloyed steels) of equal tensile strength. The fatigue strength increases with increasing yield strength. For notched specimens it is also related to the yield ratio. Work and bake hardening increase the fatigue strength of smooth specimens in proportion to the increase in yield strength. For notched specimens this effect is less and is dependent on the yield ratio. Bake hardening of material which was not work hardened also increased the fatigue strength. The notch sensitivity of low yield ratio dual-phase steel is found to be low. The notch sensitivity seems to increase with increasing yield ratio.     

不同强度混凝土高温下动态劈拉性能研究

采用大直径分离式霍普金森压杆(SHPB)装置和加温装置,在不同温度(20 ℃、200 ℃和400 ℃)下对3种不同强度(C20、C45和C70)混凝土材料开展不同应力率的动态劈裂拉伸实验,得到了温度和应力率耦合作用下混凝土材料的动态劈裂强度及相应的破坏形态。实验结果表明,混凝土材料的动态劈裂强度随应力率的增加而增加,且应力率相近时,其动态劈裂强度随温度的增加而明显降低。在此基础上给出了描述混凝土材料在不同温度下的动态劈裂强度与应力率关系的表达式,并确定了相关材料参数。通过对不同应力率和温度耦合作用下混凝土材料的动态劈裂强度进行横向对比,发现混凝土材料动态劈裂强度的温度敏感性随应力率的增加逐渐减小,并且随着混凝土材料强度的增加其温度敏感性也逐渐变小,但是其动态劈裂强度的应力率敏感性却随着温度的升高逐渐增大。     

Effect of absorbed water,temperature and strain rate on the yield strength of two methylmethacrylate-based plastics

The yielding behaviour in compression has been determined for two poly (methylmethacrylate) (PMMA)-based materials when dry or saturated with water. Measurements were made over the temperature range 0 to 60°C and at test speeds varying by 1000-fold. Wet material, as expected, shows a lower yield strength but this cannot be predicted from the strength of dry material and the change in the glass transition temperature. Yield strengths of wet material may be just as accurately calculated from that of dry material by subtracting 10 MPa for each per cent of water absorbed. A data analysis method for superimposing yield strength values at various strain rates to different temperatures is given which does not rely on a mechanical shifting of curves.     

Additive manufacturing of carbon fiber reinforced thermoplastic composites using fused deposition modeling

Additive manufacturing (AM) technologies have been successfully applied in various applications. Fused deposition modeling (FDM), one of the most popular AM techniques, is the most widely used method for fabricating thermoplastic parts those are mainly used as rapid prototypes for functional testing with advantages of low cost, minimal wastage, and ease of material change. Due to the intrinsically limited mechanical properties of pure thermoplastic materials, there is a critical need to improve mechanical properties for FDM-fabricated pure thermoplastic parts. One of the possible methods is adding reinforced materials (such as carbon fibers) into plastic materials to form thermoplastic matrix carbon fiber reinforced plastic (CFRP) composites those could be directly used in the actual application areas, such as aerospace, automotive, and wind energy. This paper is going to present FDM of thermoplastic matrix CFRP composites and test if adding carbon fiber (different content and length) can improve the mechanical properties of FDM-fabricated parts. The CFRP feedstock filaments were fabricated from plastic pellets and carbon fiber powders for FDM process. After FDM fabrication, effects on the tensile properties (including tensile strength, Young's modulus, toughness, yield strength, and ductility) and flexural properties (including flexural stress, flexural modulus, flexural toughness, and flexural yield strength) of specimens were experimentally investigated. In order to explore the parts fracture reasons during tensile and flexural tests, fracture interface of CFRP composite specimens after tensile testing and flexural testing was observed and analyzed using SEM micrograph.     

两种硅泡沫材料耐电子束辐射性能的研究

采用傅立叶红外（FT－IR）谱仪、万能材料试验机、电子蠕变松弛试验机分析了辐照前后材料的化学结构、压缩性能和应力松弛率的变化。同时使用质谱（MS）仪确定了辐解气体产物的组分，并通过负压进系统利用气相色谱（GC）仪准确测定了气体产量。结果表明，两种硅泡沫材料经辐照后其化学结构和力学性能都发生了改变，相同条件下甲基乙烯基硅泡沫（MVSF）的力学性能受吸收剂量影响比甲基苯基乙烯基硅泡沫（MPVSF）更为显著；MVSF辐解的气体产量比MDVSF的高。综合分析结果，认为MPVSF的耐辐射性比MVSP的强，因此推荐MPVSF替代MVSF用作辐照场中的垫层材料。     

Nanostructured materials by mechanical alloying: new results on property enhancement

Mechanical attrition—the mechanical alloying or milling of powders—is a very versatile and potent method of obtaining nanocrystalline or ultrafine grain structures with enhanced properties. This article presents three examples of enhanced properties obtained by materials in which the grain size has been reduced to the nanoscale or ultrafine scale by ball milling and consolidation of powders. Very high strength/hardness—the highest hardness yet reported for crystalline Mg alloys—for a ball milled Mg₉₇Y₂Zn₁ alloy is due in part to the nanocrystalline grain structure, along with nanoscale precipitates. A ternary Cu-base alloy with a low stacking fault energy was found to have both high strength and good ductility in a nanocrystalline material synthesized by the in situ ball milling consolidation method. This is another example that shows nanocrystalline materials need not be brittle. It is shown that bulk thermoelectric materials with superior properties can be produced by the ball milling and consolidation of powders to provide an ultrafine grain structure.     

Stress states and nonlinear mechanical behavior associated with nanoindentation testing of polymers

Interest in determining material properties on the nanoscale has promoted use of nanoindentation testing as a measurement technique. Classical elasticity solution of indentation geometry has provided values of the mechanical properties for linear elastic materials. Recent attempts to apply this test technique to polymers have given indications of time dependent response in the early relaxation period. There is corresponding interest in the possibility of obtaining their nonlinear viscoelastic behavior. As a preliminary to analytical study providing a basis for such testing, the first part of this paper examines the initial stress and strain condition in the vicinity of the indenter. Data from recent tests on poly(vinyl acetate) material at load levels typical of current testing indicate that stress magnitudes in the nonlinear and possibly plastic-like range are present near the specimen surface. The second part of this study pursues the examination of how the heavily nonlinear region may be characterized for polymers in analogy with the treatment utilized for metals and other elastic-plastic materials. As an example, analysis of data on PVAc indicates that its behavior in nanoindentation should in several respects correspond to materials exhibiting a relatively low value of the ratio of elastic modulus to yield stress.     

Surface carburised AISI 8620 steel with dual phase core microstructure

Abstract

In this study, the production of dual phase steel structure in the core of surface carburised AISI 8620 cementation steel and the effect of martensite volume fraction on tensile properties have been investigated. For these purposes, surface carburised (~0·8 wt-%C) specimens were oil quenched from 900°C to obtain a fully martensitic starting microstructure. Then specimens were oil quenched from intercritical annealing temperatures of 731 or 746°C to produce dual phase steel structure in the core of specimens with martensite fractions of ~25 or ~50 vol.-% and nearly wholly martensitic microstructure at the surface. Generally, specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strengths but superior ductility without sacrificing surface hardness than those specimens with fully martensitic microstructure in the core produced by using conventional heat treatment involving quenching from 850 to 950°C. Also tensile strength increased and ductility decreased with increasing martensite volume fraction.     

Free nitrogen effect on creep failure and creep crack growth of C-Mn steel

Different chemical compositions and heat-treatments of the C-Mn steels were investigated to characterise a susceptibility to cracking on cold bent tube. The C-Mn steels were categorised in terms of the content of free nitrogen. Ultimate tensile strengths, yield strengths and elongations were measured from the tensile testing at ambient, 250 and 360°C. Significant increase in ultimate tensile strength and decrease in elongation were observed in high free nitrogen material at 250°C the temperature at which free nitrogen is most active. From the uniaxial creep testing on high free nitrogen materials at 360°C increase in creep property, for example, lower minimum creep rate, was observed, however there was a decrease in creep ductility. This lower ductility of the high free nitrogen material has provided higher susceptibility to cracking in the creep crack growth tests at 360°C. Cracking in the high free nitrogen material A was approximately three times faster than the low free nitrogen material C at the same C* value. The creep cracking and rupture life in the high free nitrogen materials were more sensitive to the material condition, for example, pre-straining and/or heat treatment because of the role of the free nitrogen in the steels.     

Source truncation and exhaustion: insights from quantitative in situ TEM tensile testing

A unique method for quantitative in situ nanotensile testing in a transmission electron microscope employing focused ion beam fabricated specimens was developed. Experiments were performed on copper samples with minimum dimensions in the 100-200 nm regime oriented for either single slip or multiple slip, respectively. We observe that both frequently discussed mechanisms, truncation of spiral dislocation sources and exhaustion of defects available within the specimen, contribute to high strengths and related size-effects in small volumes. This suggests that in the submicrometer range these mechanisms should be considered simultaneously rather than exclusively.     

Electron-beam irradiation effects on mechanical properties of PEEK/CF composite

Carbon fibre-reinforced composite (PEEK/CF) using polyarylether-ether-ketone (PEEK) as a matrix material was prepared and electron-beam irradiation effects on the mechanical properties at low and high temperatures were studied. The flexural strength and modulus of the unirradiated PEEK/CF were almost the same as those of carbon fibre-reinforced composites with epoxide resin. The mechanical properties at room temperature were little affected by irradiation up to 180 MGy, but in the test at 77 K the strength of the specimens irradiated over 100 MGy were slightly decreased. The mechanical properties of the unirradiated specimen decreased with increasing testing temperature, but the high-temperature properties were improved by irradiation, i.e. the strength measured at 413 K for the specimen irradiated with 120 MGy almost reached the value for the unirradiated specimen measured at room temperature. It was apparent from the viscoelastic measurement that the improvement of mechanical properties at high temperature resulted from the high-temperature shift of the glass transition of the matrix PEEK caused by radiation-induced cross-linking.     

Effect of prestress on low-cycle fatigue

An investigation was conducted on HY-80 steel to show how low-cycle fatigue life is influenced by imposition of various stresses prior to fatigue testing. One cycle of static prestress was imposed on each specimen at loads either above or below the yield strength of the material and fatigue testing was done at various levels of stresses. Tests were conducted on notched and tee-fillet welded beam specimens. The magnitude and type of residual stresses at the toe of tee-fillet welds were determined by a hole drilling technique. Experimental data show that residual stresses induced by prestressing can—according to their sign—influence fatigue life. It was observed in the notched beam tests that when stress cycling was performed at stress levels below the yield strength of the material, tensile prestress increased fatigue life and that the higher the prestress, the greater the increase in life. At stress levels above the yield strength, tensile prestress has little or no effect on fatigue life. Compressive prestresses were found to be detrimental to fatigue life regardless of the stress levels at which the fatigue tests were run. The effects of prestress on the tee-weldments were not delinated as clearly. Fatigue life was increased by tensile prestress but no adverse effects were observed for compressive prestress.     

Application of Fracture Mechanics Concepts to Hierarchical Biomechanics of Bone and Bone-like Materials

Fracture mechanics concepts are applied to gain some understanding of the hierarchical nanocomposite structures of hard biological tissues such as bone, tooth and shells. At the most elementary level of structural hierarchy, bone and bone-like materials exhibit a generic structure on the nanometer length scale consisting of hard mineral platelets arranged in a parallel staggered pattern in a soft protein matrix. The discussions in this paper are organized around the following questions: (1) The length scale question: why is nanoscale important to biological materials? (2) The stiffness question: how does nature create a stiff composite containing a high volume fraction of a soft material? (3) The toughness question: how does nature build a tough composite containing a high volume fraction of a brittle material? (4) The strength question: how does nature balance the widely different strengths of protein and mineral? (5) The optimization question: Can the generic nanostructure of bone and bone-like materials be understood from a structural optimization point of view? If so, what is being optimized? What is the objective function? (6) The buckling question: how does nature prevent the slender mineral platelets in bone from buckling under compression? (7) The hierarchy question: why does nature always design hierarchical structures? What is the role of structural hierarchy? A complete analysis of these questions taking into account the full biological complexities is far beyond the scope of this paper. The intention here is only to illustrate some of the basic mechanical design principles of bone-like materials using simple analytical and numerical models. With this objective in mind, the length scale question is addressed based on the principle of flaw tolerance which, in analogy with related concepts in fracture mechanics, indicates that the nanometer size makes the normally brittle mineral crystals insensitive to cracks-like flaws. Below a critical size on the nanometer length scale, the mineral crystals fail no longer by propagation of pre-existing cracks, but by uniform rupture near their limiting strength. The robust design of bone-like materials against brittle fracture provides an interesting analogy between Darwinian competition for survivability and engineering design for notch insensitivity. The follow-up analysis with respect to the questions on stiffness, strength, toughness, stability and optimization of the biological nanostructure provides further insights into the basic design principles of bone and bone-like materials. The staggered nanostructure is shown to be an optimized structure with the hard mineral crystals providing structural rigidity and the soft protein matrix dissipating fracture energy. Finally, the question on structural hierarchy is discussed via a model hierarchical material consisting of multiple levels of self-similar composite structures mimicking the nanostructure of bone. We show that the resulting “fractal bone”, a model hierarchical material with different properties at different length scales, can be designed to tolerate crack-like flaws of multiple length scales.     

Fatigue properties of specimens similar to components deep rolled under optimized conditions

A method is presented for determining the rolling load that produced optimum fatigue strength improvement in deep-rolled specimens. Based on the surface stresses calculated using Hertzian theory and von Mises distortion energy failure criterion, the method yields an equivalent stress that is shown to be a suitable parameter for describing deep-rolling conditions. In addition, fatigue tests have been carried out on deep-rolled smooth and notched specimens of 37CrS4 steel. Specimens with optimized deep rolling are found to attain higher bending fatigue strengths than deep-rolled smooth samples, thus fully removing the notch effect. The greater fatigue strength improvement observed for notched samples is explained on the basis of fatigue behaviour. Adequate static strengths are demonstrated by fatigue-cracked specimens, particularly for ductile material conditions.     

Post-impact behaviour of aerospace composites for high-temperature applications: experiments and simulations

The post-impact performance of different carbon-fabric-reinforced composite materials were studied experimentally and analytically. Three types of thermosetting matrix were considered: conventional epoxy, high-temperature curing epoxy and epoxy-isocyanate. Experimental testing consisted of impacting rectangular specimens at different energy levels by using a spring-driven impact apparatus that was able to impart velocities of up to 5 m s⁻¹ to masses of 0.5, 1.0, 2.5 and 5.0 kg travelling horizontally. After impact, coupons were non-destructively inspected by means of opaque-enhanced dye-penetrant X-radiography and tested in static compression to correlate impact energy, damage extent and residual strength. Epoxy composites contain damage within a narrow region, while epoxy-isocyanate materials propagate the damage far away from impact point. Epoxy composites show an asymptotically decreasing failure strength with impact energy up to a lower threshold (0.3–0.4 times that of the undamaged material), while epoxy-isocyanate material shows a trend of ever decreasing residual strength. An analytical study was performed by means of the finite element code PAM-FISS, used to simulate the compression-after-impact (CAI) tests. Type, size and location of damage, as well as the mechanisms leading to final failure, were reproduced quite well by the finite element analysis (FEA), while some discrepancies between FEA and experimental CAI residual strength tests were found (7% for undamaged specimens and 10% for blister-delaminated specimens); higher errors were found in the case of completely delaminated specimens, mainly owing to the inability of the present software and hardware to conveniently model the complete state of damage.